Employee plans - Components of Net Periodic Benefit Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plans | U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.0	$ 0.0	$ 0.0
Interest cost	186.6	211.3	231.3
Expected return on plan assets	(230.6)	(235.2)	(253.1)
Amortization of prior service credit	(2.5)	(2.5)	(2.5)
Recognized net actuarial loss	125.1	126.4	116.0
Curtailment gain	0.0	0.0	0.0
Settlement loss	0.0	0.0	0.0
Net periodic pension cost (income)	78.6	100.0	91.7
Pension Plans | International Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	3.2	5.1	7.4
Interest cost	67.3	72.8	87.8
Expected return on plan assets	(114.4)	(127.5)	(139.5)
Amortization of prior service credit	(3.7)	(2.4)	(3.0)
Recognized net actuarial loss	42.3	49.8	40.3
Curtailment gain	0.0	(5.4)	(2.0)
Settlement loss	6.4	0.0	0.0
Net periodic pension cost (income)	1.1	(7.6)	(9.0)
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.6	0.5	0.4
Interest cost	4.8	5.6	6.2
Expected return on plan assets	(0.4)	(0.5)	(0.4)
Amortization of prior service credit	(1.6)	(0.7)	0.0
Recognized net actuarial loss	1.0	0.8	0.5
Net periodic pension cost (income)	$ 4.4	$ 5.7	$ 6.7